A condyle is the rounded projection at the end of a bone, such as the femur. Condyles may be found in pairs and define rotational bearing surfaces for the joint providing movement mostly in one place (sagittal). Each condyle is shaped generally like a partial disk, with a rounded perimeter, and an outwardly projecting bulge known as the `epicondyle`.
When a surgeon replaces a joint, such as at the knee, the surgeon will generally reference location of the dissected prosthetic and the prosthetic joint around and to -cover the dissected condylar elements. This is done so that the joint will flex sufficiently to provide a functional angle of motion that is some 90.degree. and reproducible. It is known that ideally, the prosthesis placement ought to be referenced to the transverse axis of the joint, but defining this axis in an accurate reproducible way is generally not possible. Moreover, the transverse axis of the joint is not stationary, but will vary with the state of flexion of the joint. At the knee, however, the transverse axis for motion is approximate with a line joining the epicondyles. An objective therefore is to locate the prosthetic joint appropriately about this transepicondylar line (axis). The problem with this approach has been that the epicondyles are somewhat rounded, making it difficult for the surgeon to accurately locate the most prominent outer aspects of each epicondyle with certainty by viewing the dissected joint. Moreover, the outer aspects of the epicondyles are not reliably visible on frontal or lateral radiographs, because the knee cannot be positioned to radiograph the epicondyles in profile due to overlap of condylar bone. Furthermore, the epicondyles are not readily palpable by hand through the skin. The exception for this is the elbow.
The object of the present invention, therefore, is to provide a method and apparatus for reliably and reproducibly locating the most prominent outer aspects of the epicondyles of a bone such as the femur. This permits a surgeon or technician to locate the transepicondylar line (TEL) of the joint, and use this as the reference the transverse axis of motion about which the location of the condylar portion of the joint prosthesis is then located. The resultant implant is thereby more anatomically located, and capable of flexion without excessive strain or stress on any surrounding tissue usually for more than 90.degree.. A collateral benefit of the method of the present invention is that it provides a wealth of preoperative data to an orthopaedic surgeon, who is then given the opportunity to order precisely sized joint prosthetics well in advance of surgery. This permits an institution to maintain a smaller inventory of joint prosthetics, at a fairly low cost.
In a broad aspect, then, the present invention relates to a method for determining the transepicondylar line of a bone exhibiting condyles and therefore locating the transverse axis of rotation comprising the steps of: (a) conducting a computer tomographic (CT) scan from the end of the condyles inwardly, transverse to the anatomical axis of the bone with the bone facing forwardly; (b) selecting the CT slice image exhibiting maximum epicondylar width and establishing a line between the epicondyles at the widest extent; (c) establishing a line joining the posterior tangents of the condyles on said CT image; (d) establishing measured lines from the posterior condylar tangent line at right angles to the said line between said epicondyles.
In another broad aspect, the present invention relates to a device for locating the epicondyles on a bone that has been prepared by the insertion of a intramedullary rod along the anatomical axis thereof comprising: (a) a body bracket for fixation to said intramedullary rod, against the distal end of said bone; (b) a foot plate depending from said body bracket, adjustable to bear against the posterior surface of a condyle, to fix said body bracket in a -known orientation; (c) a measuring arm extending in the direction of said bone from said bracket, adjustable according to imaging data to locate and permit the marking of a said epicondyle.